Low density cold rolled and annealed steel sheet, method of production thereof and use of such steel to produce vehicle parts

Abstract

To provide a steel sheet presenting a relative density below 7.3, an ultimate tensile strength of at least 600 MPa, and a uniform elongation of at least 17.5%.SOLUTION: A steel sheet having, in mass%, C: 0.12-0.25%, Mn: 3-10%, Al: 3.5-6.5%, P: ≤0.1%, S: ≤0.03%, N: ≤0.1%, and optionally one or more selected from Si: ≤2%, Nb: 0.01-0.03%, Ti: 0.01-0.2%, Mo: ≤0.5%, Cr: ≤0.6%, Cu: 0.01-2.0%, Ni: 0.01-3.0%, Ca: ≤0.005%, B: ≤0.01%, Mg: ≤0.005%, Zr: ≤0.005%, and Ce: ≤0.1%, the balance including iron, and having a microstructure comprising 60-90% of δ-ferrite, 8-30% of residual austenite having an average grain size of 0.6-2 μm, 1.0-10% of α-ferrite having an average grain size of 0.6-2 μm, and 0-2% of kappa precipitates (Fe,Mn) 3AlCx, where x is less than 1.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a low density steel sheet, in particular a dual phase microstructure.The steel sheet according to the invention is particularly well suited for the manufacture of inner or outer panels for vehicles such as land vehicles. [Background technology]

[0002] Environmental regulations force car manufacturers to continually reduce the CO2 emissions of their vehicles. To do so, they have several options, the main ones being to reduce the weight of the vehicle or to improve the efficiency of the engine system. Progress is often achieved by combining these two approaches. This invention is concerned with the first option, i.e., reducing the weight of automobiles. In this very specific field, there are two (two-track) options:

[0003] The first method consists in reducing the thickness of the steel while increasing the level of mechanical strength. Unfortunately, this solution has its limitations due to the significant reduction in stiffness of certain car components and the appearance of acoustic problems that create uncomfortable conditions for passengers, not to mention the inevitable loss of ductility associated with increasing mechanical strength.

[0004] The second method involves reducing the density of steel by alloying it with other lighter metals: these alloys, with lower densities, offer attractive mechanical and physical properties while allowing for a significant reduction in weight.

[0005] In particular, EP 3421629 describes a method for producing a melt with the following composition: 0.05-0.50 wt% C, 0.05-8.0 wt% Mn, 0.05-6.0 wt% Al_tot, 0.0001-0.05 wt% Sb, 0.0005-0.005 wt% Σ(Ca+REM), 5-100 ppm N, 0-2.0 wt% Si, 0-0.01 wt% S, 0-0.1 wt% P, 0-1.0 wt% Cr, 0-2.0 wt% Ni, 0-2.0 wt% Cu, 0-0.5 wt% Mo, 0-0.1 wt% V, 0-50 ppm B, 0-0.10 wt% Cr. % Ti, and having a bimodal grain microstructure consisting of a ferrite matrix composed of delta-ferrite and alpha-ferrite, the delta-ferrite having a grain size between 5 and 20 μm, the alpha-ferrite having a grain size of up to 3 μm, and a second phase consisting of one or more of bainite, martensite, and retained austenite having a grain size of up to 3 μm. However, the steel of EP 3421629 does not exhibit a low density steel, nor does it contain hard phases such as martensite and bainite. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent Application Publication No. 3421629 Summary of the Invention

[0007] It is therefore an object of the present invention to provide a steel sheet having a relative density of less than 7.3, an ultimate tensile strength of at least 600 MPa and a uniform elongation of at least 17.5%.

[0008] In a preferred embodiment, the steel sheet according to the invention exhibits a relative density of less than or equal to 7.2 and a yield strength of at least 450 MPa. DETAILED DESCRIPTION OF THE INVENTION

[0009] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0010] The carbon content is 0.12% to 0.25% by weight, and more preferably 0.13% to 0.2% by weight. Carbon is a gamma-forming element that plays an important role in the formation of retained austenite and also imparts strength and ductility. The carbon content is preferably 0.13% to 0.2%, which simultaneously achieves high strength, elongation, and stretch flangeability.

[0011] The manganese content is 3% to 10% by weight. Manganese is an important alloying element in this system, primarily due to the fact that alloying with very high amounts of manganese stabilizes austenite up to room temperature, which can help achieve target properties such as elongation and yield strength. Manganese, together with carbon, controls the formation of carbides at grain boundaries at high temperatures, thereby controlling red shortness. If manganese is present in excess of 10%, manganese can lead to central segregation, which is detrimental to the ductility of the steel of the present invention. If present at less than 3%, manganese does not stabilize the retained austenite at room temperature in adequate amounts. The preferred limit for the presence of manganese is 4% to 9%, more preferably 4% to 8%.

[0012] The aluminum content is 3.5% to 6.5% by weight. The addition of aluminum to the steel of the present invention effectively reduces its density. Aluminum is an alpha-generating element and therefore tends to promote the formation of ferrite, especially delta ferrite. Aluminum has a relative density of 2.7, which affects mechanical properties. As the aluminum content increases, dislocation mobility decreases, resulting in a decrease in uniform elongation, but mechanical strength and elastic limit also increase. Below 3.5%, the density reduction due to the presence of aluminum becomes less beneficial. Above 6.5%, the presence of ferrite increases beyond expected limits, adversely affecting the present invention. Additionally, the presence of Al greater than 6.5% can form intermetallic compounds, such as Fe-Al, Fe3-Al, and other (Fe,Mn)Al intermetallic compounds, which can cause cracking of the steel during cold rolling and introduce embrittlement into the product, which may be detrimental to the toughness of the steel. Preferably, the aluminum content is limited to strictly less than 6.5% to prevent the formation of brittle intermetallic precipitates, so the preferred limit is 4% to 6%, more preferably 5% to 6%.

[0013] Silicon is an optional element that reduces the density of steel and is effective in solid solution hardening. Nevertheless, its content is limited to 2% by weight because above that level, this element tends to form strong, adhesive oxides that cause surface defects. The presence of surface oxides can impair the wettability of the steel and potentially cause defects during hot-dip galvanizing operations. Therefore, it is preferable to limit the Si content to less than 1.5%.

[0014] Sulfur and phosphorus are impurities that weaken grain boundaries and their respective contents must not exceed 0.03% and 0.1% by weight in order to maintain sufficient hot ductility.

[0015] The nitrogen content must be less than 0.1 wt. % to prevent the formation of volume defects (blisters) during precipitation and solidification of AlN.

[0016] Niobium may be added as an optional element to the steel of the present invention in amounts of 0.01 to 0.03 wt. % to provide grain refinement. Grain refinement allows for a balance between strength and elongation. However, niobium tends to retard recrystallization during hot rolling and annealing, so the limit is kept to 0.03%.

[0017] Titanium can be added to the steel of the present invention as an optional element in amounts of 0.01 to 0.2% by weight for grain refinement in a manner similar to that of niobium.

[0018] Copper can be added as an optional element in amounts of 0.01% to 2.0% by weight to increase the strength of steel and improve corrosion resistance. A minimum of 0.01% is required to achieve this effect. However, if its content exceeds 2.0%, copper may deteriorate the surface morphology.

[0019] Nickel can be added as an optional element in amounts of 0.01 to 3.0% by weight to increase the strength and toughness of steel. A minimum of 0.01% is required to achieve this effect. However, if the nickel content exceeds 3.0%, nickel will cause a decrease in ductility.

[0020] Molybdenum is an optional element present in the steel of the present invention in an amount of 0% to 0.5% by weight. Molybdenum plays an effective role in improving hardenability and hardness when added in an amount of at least 0.01%. Mo is also beneficial to the toughness of hot-rolled products, facilitating manufacturing. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit for molybdenum is 0% to 0.4%, more preferably 0% to 0.3%.

[0021] Chromium is an optional element in the steel of the present invention, and is present in the range of 0% to 0.6% by weight. Chromium provides strength and hardening to the steel, but if used in excess of 0.5%, it will impair the surface finish of the steel. The preferred limit for chromium is 0.01% to 0.5%, more preferably 0.01% to 0.2%.

[0022] Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions by weight: Ce≦0.1%, B≦0.01%, Ca≦0.005%, Mg≦0.005 and Zr≦0.005. These elements make it possible to refine the ferrite grains during solidification, up to the maximum content levels indicated.

[0023] Furthermore, some trace elements such as Sb and Sn are obtained from the processing of the steel. The maximum limit at which these elements can be tolerated without adversely affecting the steel of the present invention is 0.05% cumulatively or singly. With the steel of the present invention, it is preferable to keep the content of these elements as low as possible, preferably below 0.03%.

[0024] The microstructure of the steel sheet according to the present invention comprises, by area fraction, 60% to 90% delta ferrite, 1% to 10% alpha ferrite, and 8% to 30% retained austenite, and optionally 0% to 2% kappa precipitates.

[0025] The delta ferrite matrix is ​​present as the primary phase in the steel of the present invention, with an area fraction of 60% to 90%, preferably 65% ​​to 90%, and more preferably 80% to 90%. Delta ferrite is formed during the solidification of slabs from liquid iron and generally has a coarse grain size. The delta ferrite of the present invention preferably has an average grain size of less than 10 μm, more preferably less than 9 μm. The presence of the delta ferrite matrix in the present invention imparts strength to the steel. However, delta ferrite contents greater than 90% in the present invention may have a negative effect due to the fact that carbon solubility increases in ferrite with increasing temperature. However, carbon in solid solution reduces dislocation mobility, making low-density steels highly embrittling, which is already low due to the presence of aluminum. Therefore, the balance between the delta ferrite content and austenite is crucial to imparting the required mechanical properties to the present invention.

[0026] The retained austenite is present in the steel of the present invention in an amount of 8 to 30%, and the retained austenite of the present invention has an average grain size of 0.6 microns to 2 microns. The preferred average grain size of the retained austenite is between 0.6 microns and 1.2 microns. Retained austenite is known to have a higher carbon solubility than ferrite and acts as an effective carbon trap. The carbon fraction in austenite is 0.7% to 1.5% by weight. Austenite present at levels greater than 30% adversely affects the present invention by impairing stretch flangeability. Austenite contributes to the present invention in a very versatile manner, depending on the annealing and compositional selection of the steel. The austenite of the present invention exhibits various functions, such as providing formability and ductility through the TRIP effect. The preferred limit for the retained austenite is an area fraction of 9% to 29%.

[0027] The alpha ferrite of the present invention is present in an area fraction of 1% to 10%. The alpha ferrite is formed by partial transformation of austenite during cooling after hot rolling and intercritical annealing, and has an average grain size of 0.6 microns to 1.85 microns. The preferred average grain size of alpha ferrite is 0.6 microns to 1.2 microns. The alpha ferrite of the present invention imparts ductility and elongation to the steel. The preferred limit of alpha ferrite is an area fraction of 2% to 10%.

[0028] The kappa precipitates in the present invention have a stoichiometry of (Fe,Mn)3AlC x where x is strictly defined by precipitates with x strictly less than 1. The area fraction of kappa precipitates can range up to 2%. Above 2%, ductility decreases and uniform elongation greater than 17.5% is not achieved. Furthermore, uncontrolled precipitation of kappa around ferrite grain boundaries can occur, which can increase the applied forces during hot and / or cold rolling. Preferentially, the area fraction of kappa precipitates should be less than 1%.

[0029] In addition to the above microstructure, the microstructure of low density cold rolled annealed steel is free of microstructural elements such as pearlite, bainite and martensite.

[0030] The steel sheet according to the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art, but it is preferred to use the method according to the invention, which comprises the following steps:

[0031] The steel sheet according to the invention is preferably produced by casting a semi-finished product of the steel according to the invention, for example a slab, thin slab or strip, having the composition described above, and first cooling the casting input stock to room temperature and then reheating it to a temperature above 1000° C., preferably above 1150° C., more preferably above 1200° C., or the cast semi-finished product can be used directly at such temperatures without intermediate cooling. The semi-finished product for the purposes of the process of the invention is considered to be a slab.

[0032] The reheated slabs are then hot rolled to a finishing temperature of above 750°C, preferably above 770°C.

[0033] After hot rolling, the strip must be coiled at a temperature below 720°C, preferably between 350°C and 720°C, more preferably between 700°C and 400°C.

[0034] The hot rolled strip is cooled to room temperature and then pickled or subjected to any other descaling process.

[0035] Next, this hot-rolled steel strip is subjected to cold rolling at a reduction ratio of 30% to 90%, preferably 40% to 90%.

[0036] After cold rolling, the cold rolled steel strip is annealed by heating it to an annealing temperature comprised between 840°C and 1000°C, preferably between 850°C and 975°C, more preferably between 850°C and 925°C, at a heating rate of at least 1°C / s, preferably more than 3°C / s, holding the strip at said annealing temperature for less than 1000 seconds, preferably less than 600 seconds, and cooling the strip at a rate of at least 3°C / s, more preferably at least 5°C / s, even more preferably at least 10°C / s. Preferably, this annealing is carried out continuously.

[0037] By controlling the annealing temperature and time, a two-phase structure can be obtained during soaking.

[0038] After such an annealing step, the steel sheet can be cooled to a temperature between room temperature and 480°C, and optionally held at 100°C to 480°C for overaging for 1 hour or less, preferably less than 20 minutes, more preferably less than 10 minutes, after which the steel sheet can be cooled to room temperature.

[0039] After annealing, the steel sheet can optionally be subjected to a metal coating operation to improve its protection against corrosion. The coating process used can be any process adapted to the steel of the invention. Electrolytic or physical vapor deposition can be mentioned, with particular emphasis being placed on jet deposition. The metal coating can be based, for example, on zinc or aluminum.

[0040] Preferably, the aluminum-based coating contains less than 15% Si, less than 5.0% Fe, optionally 0.1% to 8.0% Mg, and optionally 0.1% to 30.0% Zn, the balance being Al.

[0041] Advantageously, the zinc-based coating comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the balance being Zn. [Example]

[0042] The following tests, examples, illustrative examples and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0043] The steel sheets produced with different compositions are summarized in Table 1. For all steels, the presence of phosphorus was always less than 100 ppm, and the steel sheets were produced according to the process parameters specified in Table 2. Table 3 then summarizes the microstructures of the steel sheets obtained during the tests, and Table 4 summarizes the evaluation results of the properties obtained.

[0044] [Table 1]

[0045] [Table 2]

[0046] The obtained samples were then analyzed and the corresponding microstructural elements and mechanical properties were summarized in Tables 3 and 4, respectively.

[0047] Table 3 summarizes the results of tests performed according to standards using different microscopes, such as EBSD, XRD, or other microscopes, to determine the microstructural composition of both the inventive steel and the reference test. Using EBSD, the area fractions of delta ferrite and alpha ferrite are measured. For a given steel sample, EBSD analysis of at least four images corresponding to a magnification of 1000x makes it possible to identify ferrite grains, their location, and size. All grains with a grain size below the cutoff value of 1.85 μm and adjacent to austenite grains are counted as alpha ferrite, and the corresponding area fraction of such grains is determined. The remaining ferrite grains are counted as delta ferrite, and the corresponding area fraction of such grains is determined. Using EBSD, the average grain sizes of delta ferrite, retained austenite, and alpha ferrite are also measured. Using XRD, the retained austenite area fractions are measured and are shown in Table 3.

[0048] [Table 3]

[0049] From the above table it can be seen that the test according to the present invention meets all microstructure targets.

[0050] Table 4 summarizes the mechanical and surface properties of both the inventive and reference steels.

[0051] <Table 4: Mechanical properties of the test> The yield strength YS, tensile strength TS and uniform elongation UE are measured according to ISO standard ISO 6892-1 published in October 2009.

[0052] To determine the relative density of steel, the volume of a steel sample is measured by gas displacement pycnometry using helium on one side and its corresponding mass is measured on the other side. It is then expressed in g / cm. 3 Calculate the mass per volume ratio of this steel, expressed as 1 g / cm 3Divide by the mass per volume ratio of water at 4°C, which is equivalent to the relative density of steel. The resulting value (unitless) is the relative density of steel.

[0053] [Table 4]

[0054] The examples show that the steel sheets according to the invention are unique in that they exhibit all the target properties thanks to their specific composition and microstructure.

Claims

1. 1. A low density cold rolled annealed steel sheet having, by weight, the following properties: 0.12%≦carbon 0.25%; 3%≦Manganese≦10%, 3.5%≦aluminum≦6.5%, 0%≦phosphorus≦0.1%, 0%≦sulfur≦0.03%, 0%≦nitrogen≦0.1%, and optionally the following elements: 0%≦silicon≦2%; 0.01%≦niobium≦0.03%, 0.01%≦Titanium≦0.2%, 0%≦molybdenum≦0.5%, 0%≦chromium≦0.6%, 0.01%≦copper≦2.0%, 0.01%≦Nickel≦3.0%, 0%≦Calcium≦0.005%, 0%≦boron≦0.01%, 0%≦Magnesium≦0.005%, 0%≦zirconium≦0.005%, 0%≦Cerium≦0.1% and the balance being iron and unavoidable impurities, the steel sheet having, by area fraction, 60% to 90% delta ferrite, 8% to 30% retained austenite having an average grain size between 0.6 and 2 microns, 1.0% to 10% alpha ferrite having an average grain size between 0.6 and 1.85 microns, 0% to 2% kappa precipitates (Fe, Mn), where x is strictly less than 1. ３ AlC ｘ A low-density cold-rolled annealed steel sheet having a microstructure comprising:

2. 2. The steel sheet according to claim 1, wherein the carbon content is comprised between 0.13% and 0.2%.

3. 3. The steel sheet according to claim 1, wherein the manganese content is between 4% and 9%.

4. 4. The steel sheet according to claim 1, wherein the content of retained austenite is between 9 and 29%.

5. A steel sheet according to any one of claims 1 to 4, wherein the content of kappa precipitates is between 0% and 1%.

6. Steel sheet according to any one of the preceding claims, having an alpha-ferrite content between 2% and 10% and an average grain size between 0.6 and 1.2 microns.

7. The steel sheet according to any one of claims 1 to 6, which is covered with a metal coating.

8. A method for manufacturing a steel sheet, comprising the steps of: - providing a slab of the composition according to claims 1 to 3, - reheating the slab to a temperature above 1000°C and hot rolling it to a final rolling temperature of at least 750°C; - coiling the hot-rolled steel sheet at a temperature below 720°C, - cooling the hot-rolled sheet; - pickling the hot-rolled steel sheet; - cold rolling the hot-rolled steel sheet at a reduction ratio of 30% to 90% to obtain a cold-rolled steel sheet; - annealing the cold rolled steel sheet by heating it from room temperature to an annealing temperature of between 840°C and 1000°C at a heating rate of at least 1°C / s; - then annealing for less than 1000 seconds, - cooling the cold rolled steel sheet to a cooling stop temperature of 480°C to room temperature at a cooling rate of at least 3°C / s, optionally holding the cold rolled steel sheet between 100°C and 480°C for 1 to 200 seconds; - thereafter cooling the cold rolled steel sheet to room temperature to obtain a low density cold rolled annealed steel sheet. A method comprising:

9. The method according to claim 8, wherein the annealing temperature is comprised between 850°C and 975°C.

10. 10. The method according to claim 8 or 9, wherein the coiling temperature is comprised between 350°C and 720°C.

11. The method according to any one of claims 8 to 10, wherein the holding time of the annealing is less than 600 seconds.

12. The method according to any one of claims 8 to 11, wherein the heating rate of the annealing is greater than 3°C / s.

13. The method of any one of claims 8 to 12, further comprising a final coating step.

14. Use of a steel sheet according to any one of claims 1 to 7 or obtainable by the method according to any one of claims 8 to 14 for the manufacture of structural or safety parts of a vehicle.

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